Remediation of contaminated diverse natural resources such as soils, river and lake muds, land development fields, water and wastewater treatment sludges and the like poses complicated problems. Particularly, chromium ore processing residue contains extremely toxic hexavalent chromium values which present a significant disposal problem.
The hexavalent chromium contaminated soils can adversely effect the environment by dry toxic dust being blown into the air which can cause carcinogenic effects on human health, and by the discharged leachates which, when flushed by rains and other precipitations through contaminated soils, muds, sludges, etc., would contain toxic chromium and would contaminate ground water aquifers, agricultural fields and surface water resources.
The invention presents a versatile system which lends itself to be adaptable to a wide variety of different field applications. Such applications of the proposed method also is of great importance in terms of environmental protection and natural resources recovery. Industries which discharge the toxic chromium ore are held responsible for remediating soil. If such industries no longer exist, the governmental agencies are often held accountable for reducing the concentrations of toxic contaminants to acceptable leachate levels normally tested by the Toxicity Characteristic Leaching Procedure (TCLP).
The present invention addresses the need for methods for detoxifying and stabilizing dangerous chromium compounds effectively in soils, muds, sludges, and the like. It will become apparent to those skilled in the art that the method disclosed herein is not limited in application solely to detoxification and stabilization of chromium ore contaminated soil mentioned above, and that the present invention is adaptable to other remediation efforts involving the treatment of other heavy metals.
Physical processes for heavy metal, particularly chromium removal from soils are known in the art and they include flushing the contaminated solid materials then treating elutriates and stabilizing toxic metals and other inorganic elements in a stable final material.
The known processes of soil flushing suffer many drawbacks because a substantial portion of the metals, particularly chromium, in the soils are bound with organic soil components and cannot be released unless the organic compounds are destroyed. That is why the relevant industries prefer chemical methods over physical procedures for removing heavy metals, particularly chromium, and other elements from contaminated materials, but these chemical methods of metal extraction are focused on costly and complex procedures.
There is a number of known processes and apparatus for heavy metals, particularly chromium, immobilization (stabilization) in contaminated solid material, residue or waste. However, there is no evidence that available methods used for contaminated soil remediation in non-toxic forms can economically meet current United States Environmental Protection Agency (USEPA) requirements based on the standard Toxicity Characteristic Leaching Procedure (TCLP) limits.
An example of such a method is described in U.S. Pat. No. 4,504,321 (1984) which is focused on admixing chromium ore waste with mud or sludge dredged from a natural preferably salty water source, then admixing the above mixture with ground granulated blast furnace slag and finishing with setting the final admixture in a quiescent state to yield a solid mass having low permeability and high load bearing characteristics. The unsolidified admixture is basified, particularly treated by lime, to yield pH 10.0-12.0. One of major drawbacks of this invention is the necessity of a source of salty water mud or sludge which restricts the area of applicability of this method. A further drawback of this method is complexity and cost of sludge (mud) treatment by settling/dewatering as well as that of blast furnace slag handling. Another drawback of this method is a relatively high pH of the final solid product which can cause problems by using the product as backfill.
Another procedure described in U.S. Pat. No. 4,132,558 for treating a sludge or drainage containing hexavalent chromium compounds comprises the use of calcium haloaluminate bearing rapid hardening cement in a short time for Cr.sup.6+ fixation. The method is focused on hexavalent chromium fixation without reducing it to the trivalent state. The invention does not contain any information on meeting the TCLP limits, thus cannot be advantageous to the method proposed herein. Also such specific reagent as calcium haloaluminate can be expensive and is considered impractical for large amounts of soil to be treated.
A still further process for converting hazardous industrial or other wastes into an inert, non-polluting and useful soil-like product is described in U.S. Pat. No. 4,149,968. This method is based on treating the toxic wastes by an admixture of bentonite clays with portland cement in alkaline pH range to provide encapsulation of pollutants in the matrix of solid mass material. Along with a very high cost of bentonite clays, one of the most important disadvantages of this method is that the final product is liquid and requires large volumes of excavation and containment to provide hardening and handling.
U.S. Pat. No. 4,687,373 describes a coating composition for treating solid wastes to encapsulate toxic metal and/or organic pollutants from wastes includes an aqueous silicate solution containing potassium oxide and silicon dioxide, a catalytic amount of an aqueous sodium borate solution and a fixative containing solid calcium oxide with possible addition of fumed silica. Such an exotic composition was proposed for solid wastes treatment and is considered useful for small amount of a toxic metal containing material but impractical for soil remediation.
Thus, a continuing problem faced by those skilled in the art is the practical utility of incorporating above concepts into workable, efficient and feasible chemical engineering systems.